An iodide ion is an iodine atom with a −1 charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as carbon tetraiodide. This is the same naming scheme as is seen with chlorides and bromides The chemical test for an iodide compound is to acidify the aqueous compound by adding some drops of acid, to dispel any carbonate ions present, then adding lead nitrate, yielding a bright yellow precipitate of lead iodide. Most ionic iodides are soluble, with the exception of yellow silver iodide and yellow lead iodide. Iron(III) iodide does not exist because iron(III) ions oxidize iodide ions in aqueous solution. Aqueous solutions of iodide dissolve iodine better than pure water due to the formation of complex ions:
- I−(aq) + I2(s) ⇌ I3−(aq)
The colour of the triiodide ion formed is brown.
Examples or common iodides include:
- hydrogen iodide (HI)
- sodium iodide (NaI)
- potassium iodide (KI)
- carbon tetraiodide (CI4)
- silver iodide (AgI)
- nitrogen triiodide (NI3)
Iodide as an antioxidant
Iodide (>6mg/day) can be used to treat patients with hyperthyroidism due to its ability to block the release of thyroid hormone (TH), known as the Wolff-Chaikoff Effect, from the thyroid gland. In fact, prior to 1940, iodides were the predominant antithyroid agents. In large doses, iodides inhibit proteolysis of thyroglobulin. This permits TH to be synthesized and stored in colloid, but not released into the bloodstream.
Of note, this treatment is seldom used today as a stand-alone therapy despite the rapid improvement of patients immediately following administration. The major disadvantage of iodide treatment lies in the fact that excessive stores of TH accumulate, slowing the onset of action of thioamides (TH synthesis blockers). Additionally, the functionality of iodides fade after the initial treatment period. An "escape from block" is also a concern, as extra stored TH may spike following discontinuation of treatment.